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Beaches are important coastal features that provide vital ecosystem services; however, these systems are threatened by coastal erosion, sea-level rise, and coastal squeeze. Beach nourishments are a commonly applied coastal protection measure to mitigate erosion and flood risks while maintaining or enhancing recreational values. Nourishments vary in scale from mega-nourishments to small-scale nourishments, where the latter has typically been less studied due to the limited resources for monitoring. Meanwhile, with rising sea levels, the implementation of small-scale nourishments is expected to increase, and there is a need for more knowledge about the morphological evolution and technical lifetime of these interventions. In this study, the morphological responses of small-scale beach nourishment (total volume 20,000 m3 which amounts to 30 m3 added per m alongshore) are observed and quantified over various timescales, considering the initial adjustment, long-term development, and event-driven response. The investigated nourishment is implemented in a partly sheltered coastal embayment in the semi-enclosed Baltic Sea, which has a complex interaction between waves and water levels. Furthermore, the beach is surrounded by hard structures, a rock revetment at the back, and a harbor mole and a groin, influencing the longshore and cross-shore sediment transport. The results show that substantial reduction in subaerial volume can be attributed to specific events. In addition, we observed considerable spatial variation in the sediment re-distribution induced by hard structures and variability in the nearshore bathymetry. The lifetime of the beach nourishment is just over two years. The nourished material remains in the system at the end of the lifetime but is not available for beach recovery. Still, the added subaerial volume has eroded at the midsection, and the protective beach width has been reduced, leaving the rock revetment exposed with reduced protection for the hinterland. The energy conditions at the site are highly episodic, which impacts morphological evolution, and observations indicate that the development is event-driven. • Beach morphodynamics in a low-energy coastal embayment • Episodic wave activity and water level variations are influenced by basin effects. • Morphological response of a small-scale beach nourishment at different timescales • Beach nourishment performance during an extreme storm surge [ABSTRACT FROM AUTHOR]

While the impact of the spatio/temporal variability of grain size on morphological beach state is reasonably well understood, relatively little is known on its impact at a scale of days/months on aeolian sediment transport. This study focuses on five short intensive wind events during which aeolian sediment transport measurements, beach surface sampling and elevation change surveys were carried out for 1 to 3 days, over a 16-month period on a microtidal beach dominated by offshore winds. Monthly observations show a high temporal variability in beach grain size in relation to the decoupling between hydrodynamic and aeolian processes, from medium sand after a marine storm and inundation of the beach, to very coarse sand after several weeks of storm-force winds. During each wind event, topographic change on the beach ranged from zero, to 0.55 m. The time scale of coarsening depended on the initial beach grain size and could be very fast when the beach was composed of medium sand (e.g. 388 μm sand changed to coarse sand of 547 μm in 40 h). In contrast, it took one month to transition from a coarse median beach grain size of 883 μm to a very coarse one of 1323 μm. This variability in grain size results in dramatically different rates of sediment flux. For example, during average wind speeds of 10 to 14 m/s the sediment flux when the beach was composed of medium sized sand ranged between 21 and 154 kg/m/h compared to 0.4 to 50 kg/m/h when the beach was composed of coarse and very coarse sized grains. Overall, this study highlights the importance of beach grain size variability on aeolian sediment transport and shows that for similar incident wind and climatic conditions aeolian sediment transport rates vary dramatically. The study demonstrates the importance of taking care when using a constant median grain size in the calculation of long-term aeolian sand transport on beaches with heterogeneous spatial and temporal beach grain size variability. • Transport rates can be ten times greater on medium-sized beach than coarse one. • Offshore wind generate a grain size sorting, the finest sands are exported seaward. • Topographic variations of the beach depends on its grain size and sand supply. • The return of fine sediments depends on marine storms occurrence and intensities. [ABSTRACT FROM AUTHOR]

Predicting the future behavior of beach and nearshore systems requires an accurate delineation and understanding of coastal cell boundaries, sediment transport pathways, and sediment sources and sinks. Unfortunately, there is a paucity of field datasets on beach and nearshore morphological change that extend fully from the top of the dunes to beyond the depth of closure to enable quantification of the sediment budget. Here, for the first time, we employ a total sediment budget approach, examining a sandy and embayed beach located in the north coast of SW England, to investigate inter- and multi-annual embayment scale sediment dynamics over a 10-year period that includes extreme storm erosion and post-storm recovery. We demonstrate that, despite the deeply embayed nature of the beach, the shoreline orientation roughly parallel to the dominant wave direction and the overwhelmingly cross-shore forcing of the inter-tidal beach volume, the system is neither closed, nor balanced. The very significant net changes in the recorded sediment volume from dune top to depth of closure (−14.5 m ODN), representing a loss of c. 100 m3 m−1 during the extreme storm period and a gain of c. 200 m3 m−1 during the recovery period, indicate that significant sediment transport occurs seaward of the base of the terminating headlands and beyond the morphological depth of closure. The results further indicate that the inter-tidal region is partly uncoupled from the sub-tidal region, with the former region dominated by cross-shore sediment fluxes, whereas the subtidal region is also significantly affected by longshore sediment fluxes. A conceptual model is presented that balances the observed volume changes with inferred fluxes, forced by variations in total and alongshore wave power. This study contradicts the general assumption that when sediment exits the inter-tidal, it rests undisturbed in the sub-tidal, waiting for a period of low-moderate energy to bring it onshore. The large sediment volumetric variations across the lower shoreface (depth of 5–20 m), which are of the same order of magnitude as, but uncorrelated with, those occurring in the inter-tidal region, are suggestive of an energetic longshore transport system across this deeper region. It is possible that this transport system extends along the whole north coast of SW England and this finding may lead to a shift in understanding of sediment budgets along exposed and macrotidal embayments globally. Unlabelled Image • Full sediment budget was produced along an exposed macrotidal embayment and linked to wave forcing. • Inter- and sub-tidal volumes are not inversely correlated, indicating decoupling. • Sediment budget suggests significant exchange around headlands and/or across depth of closure. • Beaches on this coast type can be considered part of an extended coastal cell. [ABSTRACT FROM AUTHOR]

The twin ambits of climate change and coastal development have raised public awareness of shoreline management. Simultaneously, they have highlighted a gap in our understanding of sediment transport and morphodynamic processes at time and space scales appropriate for shoreline management purposes. Here, we analyse an exceptional set of beach surveys gathered over a period of twenty-two years along the Suffolk coast, eastern UK, that extends over approximately 80 km to investigate the meso-scale shoreline variations. The surveys have been made biannually along fixed transects spaced at approximately 1 km intervals as part of a strategic monitoring exercise undertaken by the coastal authorities to assist in shoreline management planning. Changes in beach volume, foreshore slope and shoreline position have been computed to investigate both spatial and temporal changes. The analysis reveals some distinct responses to the physical processes of tides and waves, anthropogenic interventions and geological controls. Neither a clear relationship between the presence of sea defences and beach response nor an ordered regional-scale shoreline movement are evident. Temporal variations in beach volumes and position provide a similarly complex picture with recessionary, accretionary and stable behaviour all apparent within the study site. There is evidence of quasi-cyclic behaviour at some locations as well as a reduction in variability over time-scales beyond approximately five years. • Historic beach surveys over a period of 22 years along a frontage of 80km have been analysed for their meso-scale dynamics. • Meso-scale morphological changes of protected and unprotected beaches have been classified. • The majority of beaches fronting hard defences don't exhibit erosive trends; the majority of those without hard defences do. • Beach morphodynamics are constrained to the synoptic scale, with little coherent variation at the meso-scale. [ABSTRACT FROM AUTHOR]

Estimating sediment budget is a challenge that requires understanding the morphological relationship between the subaerial beach and the upper and lower shoreface. Based on a unique dataset of monthly measured topographic and bathymetric measurements, this study describes the morphodynamics of the subaerial and subtidal sections of a highly-dynamic beach over a 5-year period (2015–2020). Typically, the beach presents a longshore bar trough (LBT) state at the end of the winter, driven by net offshore sediment transport. The nearshore morphology evolves into more reflective states (rhythmic bar beach, RBB; transverse bar rip, TBR; and low tide terrace, LTT) driven by onshore sediment transport when the incident wave-energy decreases over the spring and summer. Seasonal variations of the incident wave-energy resulted in large subaerial-subtidal exchanges of sediment (200–300 m3 m−1), but these exchanges were an order of magnitude smaller in yearly scales. Subaerial and upper-shoreface morphological changes varied alongshore. Larger variations existed in the south, influenced by persistent rip channels. Instead, alongshore processes resulting from changes in wave direction may control the morphodynamics in the center and north. The subtidal section in north and center gained significant amount of sand (300 m3 m−1) during high-energy conditions, which contributed to a net annual gain of 60 m3 m−1. The origin of that sediment is currently unknown, it could have bypassed from an adjacent beach, shifted alongshore or inputted from the lower shoreface deeper than the depth of closure. This research shows the presence of very large monthly-to-seasonal total volume fluctuations relative to net annual or multi-annual changes, and highlights that comparatively small additions to the annual sediment budget could potentially contribute to the long-term stability of the beach. • Largest morphological variations in the south influenced by persistent rip channels. • Very large monthly-to-seasonal volume fluctuations relative to net annual changes. • Small additions to the annual budget could contribute to long-term beach stability. [ABSTRACT FROM AUTHOR]

The typically curved and stable planform shape of embayed beaches is often observed in the presence of rigid headlands. The long-term alongshore equilibrium shape is variable and is controlled by headland geometry, cellular circulation patterns and wave obliquity at the shoreline. We use a process-based morphological model to simulate the development of embayed beaches depending on different environmental conditions and geological settings (i.e., topography and geometry). The embayment is allowed to develop from an initially straight beach to one which is curved under idealized wave forcing conditions and without external sediment input. Bay development can be approximated with an exponential function with coefficients representing rate of growth and bay size. Long-term shoreline cutback is both uniform (shoreline translation) and non-uniform (shoreline rotation) and is parameterized. Alongshore gradients in flow and transport patterns are related to long-term non-uniform shoreline cutback. Rotation of the beach ceases as the bay matures, leading to a curved shoreline planform, which is a remnant of decaying erosion processes. [ABSTRACT FROM AUTHOR]

A field-based experimental study of beach scarp morphodynamic evolution was conducted on the shoreface of a macrotidal sandy beach subject to storms combined with spring tide events (Luc-sur-Mer, France). Both video and in-situ measurements on an artificial berm are used to understand beach scarp evolution over one tide during stormy conditions. Image time stacks are used to analyze the swash action on the beach scarp and topographical data of the scarp are recorded with a terrestrial scanner laser to quantify the morphodynamic response of the beach scarp to wave action. This work provides a new and unique dataset about beach scarp changes and berm morphology in particular under rising tide and oblique wind-wave conditions. During one stormy event, the berm was completely destroyed. However, contrasting alongshore changes were measured during the erosive phase with different crest and foot scarp retreats and eroded volumes between the west and the east side of the berm. The beach in front of the scarp also shows a contrasting residual evolution, indicating an evident longshore sediment transport on the study area as a consequence of incident oblique wave conditions. A strong connection between beach evolution and beach scarp changes is clearly identified. The scarp erosion increases on the west side of the berm when the beach level is lowered and reduces when the beach surface rises on the east side. The beach slope and foreshore elevation as a result of a longshore sediment transport between east and west profiles, influence swash activity. Overall, water depth and swash activity became progressively different along the scarp during the experiment. Swash measurements indicate that the presence of the beach scarp strongly influences the swash motion. At high tide, the reflection of the uprush on the scarp front induces a collision between the reflected backwash and the following uprush dynamic. These collisions reduce and sometimes stop the motion of the following uprush, reducing the incoming swash excursion. Consequently, the scarp presence modifies the swash interaction that normally appears on a planar beach surface. With a beach scarp, the swash energy level is substantially attenuated and its spectrum is characterized by a large band. The number of uprush impacts on the scarp front calculated from video images reaches about 25 per 5 min. In spite of the swash energy attenuation due to swash/swash interactions, these impacts provoke the berm destruction in about two hours. However, the onshore migration of the swash zone induced by the rising tide appears to be important to explain scarp destruction, compensating the attenuated swash activity due to backwash-uprush interactions. [ABSTRACT FROM AUTHOR]

Understanding time scales of beach response (erosion and recovery) to extreme storms is particularly relevant for management of coastal land and risk. Observations show that rock-bounded platform beaches are generally remarkably stable and only occasionally respond to extreme storm wave forcing with abrupt morphological changes. The present work aims to understand the conditions leading to the disruption of beach stability (storm thresholds for morphological changes) and to comprehend the conditions of the subsequent recovery. Results obtained at two rock-bounded beaches of the western Portuguese coast over five years provided the foundations of a new conceptual morphodynamic model. This model explains abrupt beach disruption as a consequence of full inundation of the beach profile during extreme storms, together with strong wave reflection at the rigid landward boundary, causing major erosion of the subaerial beach. We argue that the timescale of post-storm recovery depends essentially on the fate of the sediment transported offshore during the storm event. If sediments remain within the beach sediment cell (underwater beach section) beach recovery to pre-storm conditions is very fast, while recovery is much slower if sediments exit the beach sediment cell. In the latter case, beach recovery depends on external sand sources, which can lead to recovery time scales of several years. • We investigate conditions leading to abrupt morphological changes in rock-bounded platform beaches. • Total water level adequately describes thresholds for significant erosion during major storms. • A conceptual model explaining patterns of erosion and recovery is proposed. • Abrupt erosion is triggered by wave reflection at the landward hard boundary of the beach. • Beach recovery depends on the geological framework, which controls sediment exchanges with adjacent sediment cells. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes field measurements that document the formation of barchan dunes in the Moroccan Atlantic Sahara desert. The first mechanism described is the transformation of patches and proto-dunes at Cap Juby beach to barchan dunes of elementary size. This transformation is discussed in relation to the wind speed and saturation length. The second mechanism is the ejection of small barchans of elementary size by other small barchans in response to the perturbation of the target barchan by two other impacting small barchans. This remote initiation is discussed in relation to the bulk flux transported by the barchan dunes involved in this interaction and to their morphology. Other modes of barchan initiation observed in the field are also presented. [Copyright &y& Elsevier]

Abstract: Sediment supply provides a fundamental control on the morphology of river deltas, and humans have significantly modified these supplies for centuries. Here we examine the effects of almost a century of sediment supply reduction from the damming of the Elwha River in Washington on shoreline position and beach morphology of its wave-dominated delta. The mean rate of shoreline erosion during 1939–2006 is ~0.6 m/yr, which is equivalent to ~24,000 m3/yr of sediment divergence in the littoral cell, a rate approximately equal to 25–50% of the littoral-grade sediment trapped by the dams. Semi-annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer. Shoreline change and morphology also differ spatially. Negligible shoreline change has occurred updrift (west) of the river mouth, where the beach is mixed sand to cobble, cuspate, and reflective. The beach downdrift (east) of the river mouth has had significant and persistent erosion, but this beach differs in that it has a reflective foreshore with a dissipative low-tide terrace. Downdrift beach erosion results from foreshore retreat, which broadens the low-tide terrace with time, and the rate of this kind of erosion has increased significantly from ~0.8 m/yr during 1939–1990 to ~1.4 m/yr during 1990–2006. Erosion rates for the downdrift beach derived from the 2004–2007 topographic surveys vary between 0 and 13 m/yr, with an average of 3.8 m/yr. We note that the low-tide terrace is significantly coarser (mean grain size ~100 mm) than the foreshore (mean grain size ~30 mm), a pattern contrary to the typical observation of fining low-tide terraces in the region and worldwide. Because this cobble low-tide terrace is created by foreshore erosion, has been steady over intervals of at least years, is predicted to have negligible longshore transport compared to the foreshore portion of the beach, and is inconsistent with oral history of abundant shellfish collections from the low-tide beach, we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta. Thus, the cobble low-tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta, which in turn is related to the impacts of the dams on the Elwha River to sediment fluxes to the coast. [Copyright &y& Elsevier]

Abstract: Landsat Enhanced Thematic Mapper imagery (ETM) of 2002 and aerial photography of 1955, combined with published charts and field observations were used to interpret geomorphological changes in the coastal zone between Kitchener drain and Damietta spit in the northeastern Nile delta previously recognized as a vulnerable zone to the effects of any sea-level rise resulting from global warming. The interpretation resulted in recognition of several changes in nine identified geomorphological land types: beach and coastal flat, coastal dunes, agricultural deltaic land, sabkhas, fish farms, Manzala lagoon, saltpans, marshes and urban centers. Reclamation of vast areas of the coastal dunes and of Manzala lagoon added about 420 km2 to the agricultural deltaic land. About 48 km2 of sand dunes, marshes, sabkhas and salt pans have been converted to productive fish farms. The main urban centers have expanded, and new urban centers (Damietta harbor and the New Damietta city) have been constructed at the expense of vast areas of Manzala lagoon, coastal dunes, and backshore flats. As the consequence of human activities the size of Manzala lagoon has been reduced to less than 50%. Short-term shoreline changes along the coast of the study area established from beach profile survey, spanning the years of 1990 to 2000, reveal longshore patterns of erosion versus accretion. The rate of shoreline retreat has increased in the downdrift side of Damietta harbor (−14 m/year), whereas areas of accretion exist within the embayment of Gamasa and in the shadow of Ras El Bar detached breakwater system, with a maximum shoreline advance of ~15 m/year. A sandy spit, 12 km long, has developed southeast of Damietta promontory. These erosion/accretion patterns denote the natural processes of wave-induced longshore currents and sediment transport, in addition, the impact of man-made coastal protection structures. [Copyright &y& Elsevier]

Abstract: Beach profile equilibrium is the principal concept assumed by most numerical modelling. The response of beach profile configuration to natural and anthropogenic changes could be predicted to help in selecting the most appropriate engineering design required to mitigate coastal erosion and accretion alongshore. Thus, in order to apply coastal engineering projects, the predicted profile of equilibrium should be close enough to the measured profile. Therefore, before application of numerical modelling techniques, equilibrium expressions have to be validated at the study site. This research aims to assess the validity of the equilibrium profile concept on the Nile Delta coast based on beach profiles surveyed in 1990 from the main promontories; Abu-Quir Bay, the Rosetta promontory and the El-Burullus promontory. The results indicate that the equilibrium beach profile is consistent with the measured profiles at the study sites beyond 200 m distance offshore at −4 m depth. In contrast, the equilibrium status is not valid along the beach face at −1 m depth. Accreted beaches at Abu-Quir bay and Burullus promontory are characterized by wide berms and gentle beach face whereas eroded stretches at Rosetta promontory have a narrow berm and steep beach faces. The measured profiles are also compared with the exponential beach profile concept. An exponential hypothesis is not valid along the Nile Delta coast. Profiles measured at Abu-Quir, Rosetta and El-Burullus depart significantly from the exponential equation. Despite the fact that equilibrium expression can describe beach profiles along the Nile Delta, one equilibrium profile equation is not sufficient to assess all beach profiles. This can be explained as the morphology of beach profiles is subjected to some factors including; sediment characteristics, wave parameters and closure depth, which vary alongshore. Analysis of the validity of the equilibrium beach profile is recommended to get accurate results in modelling simulations and design the most appropriate engineering projects required for shore protection. [Copyright &y& Elsevier]

Abstract: The 2004 Indian Ocean tsunami devastated the coastal areas along the Andaman western coast of Thailand and left unique physical evidence of its impact, including the erosional landforms of the pre-tsunami topography. Here we show the results from monitoring the natural recovery of beach areas at Khuk Khak and Bang Niang tidal channels of Khao Lak area, Phang-nga, Thailand. A series of satellite images before and after the tsunami event was employed for calculating the beach area and locating the position of the changed shoreline. Field surveys to follow-up the development of the post-tsunami beach area were conducted from 2005 to 2007 and the yearly beach profile was measured in 2006. As a result, the scoured beach areas where the tidal channel inlets were located underwent continuous recovery. The return of post-tsunami sediments within the beach zone was either achieved by normal wind and wave processes or during the storm surges in the rainy season. Post-2004 beach sediments were derived mainly from near offshore sources. The present situation of the beach zone has almost completed reversion back to the equilibrium stage and this has occurred within 2 years after the tsunami event. We suggest these results provide a better understanding of the geomorphological process involved in beach recovery after severe erosion such as by tsunami events. [Copyright &y& Elsevier]

Abstract: Large tsunamis are major geomorphic crises, since they imply extensive erosion, sediment transport and deposition in a few minutes and over hundreds of kilometres of coast. Nevertheless, little is known about their geomorphologic imprints. The December 26, 2004 tsunami in Sumatra (Indonesia) was one of the largest and deadliest tsunamis in recorded human history. We present a description of the coastal erosion and boulder deposition induced by the 2004 tsunami in the Lhok Nga Bay, located to the West of Banda Aceh (northwest Sumatra). The geomorphological impact of the tsunami is evidenced by: beach erosion (some beaches have almost disappeared); destruction of sand barriers protecting the lagoons or at river mouths; numerous erosion escarpments typically in the order of 0.5–1.5 m when capped by soil and more than 2 m in dunes; bank erosion in the river beds (the retreat along the main river is in the order of 5–15 m, with local retreats exceeding 30 m); large scars typically 20–50 cm deep on slopes; dislodgement of blocks along fractures and structural ramps on cliffs. The upper limit of erosion appears as a continuous trimline at 20–30 m a.s.l., locally reaching 50 m. The erosional imprints of the tsunami extend to 500 m from the shoreline and exceed 2 km along riverbeds. The overall coastal retreat from Lampuuk to Leupung was 60 m (550,000 m2) and locally exceeded 150 m. Over 276,000 m3 of coastal sediments were eroded by the tsunami along the 9.2 km of sandy coast. The mean erosion rate of the beaches was ~30 m3/m of coast and locally exceeded 80 m3/m. The most eroded coasts were tangent to the tsunami wave train, which was coming from the southwest. The fringing reefs were not efficient in reducing the erosional impact of the tsunami. The 220 boulders measured range from 0.3 to 7.2 m large (typically 0.7–1.5 m), with weights from over 50 kg up to 85 t. We found one boulder, less than 1 m large, at 1 km from the coastline, but all the others were transported less than 450 m (<7 m a.s.l.). No fining landward boulder size distribution could be detected. The coincidence of different size modes, from boulders to fine sands, with independent spatial distributions, suggests that all the material was not transported in suspension, but rather by a combination of suspension and bed load transport. Finally, the spatial and size distributions of tsunami boulder deposits mostly depend on the location and characteristics of their source (coral reef, beach rock, platform, dams), together with clast and surface interference during transport. One year after, the coastal environment in northwest Sumatra is still in a post-tsunami dynamic. Thus, the difference between the largest tsunamis (height>30 m) and the moderate tsunamis (height<10 m) could be their long-term impact on coastal environments. [Copyright &y& Elsevier]

Dunes are key elements of coastal landscapes in almost every latitude. They host high levels of biodiversity and provide important benefits to society; e.g. protection against floods and erosion, or recreation. Coastal dune growth is constrained by intrinsic factors, which are critical when managing dune systems or choosing coastal dune restoration as an alternative green solution for coastal protection. Here, the evolution of a beach-dune system, characterized by a reflective coarse sandy beach and low dunes, is explored to identify the favourable and optimal conditions for dune growth in these settings. Dune growth capacity is evaluated by analysing the topographical changes observed along a coastal dune over two different temporal scales (interannual and event scale) and comparing the observations with theoretical approximations of sediment transport potentials. Observations and predictions over interannual scale document that (1) temporal variability in external conditions (wind regime) and spatial variability of estimated wind fetch length, alone, fail to explain alongshore dune growth patterns and (2) optimal conditions for dune growth occur when storms (strong winds) impact the study area, jointly with low runup levels, at zones of shoreline progradation and absence of direct human influence. Conversely, lowest values of dune accumulation are associated with areas where shoreline retreat was documented. Observations from event timescales suggest that sediment transport potential can be reached over zones with no significant signs of beach erosion, if runup levels remain low and the event duration is shorter than the time scale of sand surface depletion within the upper beach. Unlabelled Image • The growth capacity of a dune adjacent to a reflective coarse sandy beach is evaluated. • Accumulation at interannual and event scales is compared to potential transport. • Longshore variability partially associated with backshore width. • Wind intensity and shoreline trend imprint a first order signal at interannual scale. • Optimal conditions for dune growth occur if strong winds blow over a stable beach. [ABSTRACT FROM AUTHOR]

Morphological variations in a sandy-muddy transitional beach (SMT-beach) are expected to coincide with sand-mud transition (SMT) boundary migrations. SMT migrations of SMT-beaches should also differ in river estuaries and semi-enclosed bays due to their contrasting dynamic sedimentary environments. However, the anticipated diversity of SMT migrations of estuarine and bay beaches and their potential morphodynamic responses are still poorly understood. Thus, four measurements of SMT spatial distributions and typical profiles of 6 estuarine beaches and 6 bay beaches located along the macrotidal coasts of the Taiwan Strait, East Asia are presented, and these sites were studied in April and August of 2018 and 2019, respectively. The results show that the SMT boundary of the estuarine beaches continuously migrates landward with increased elevations. Simultaneously, the SMT boundary of bay beaches synchronously remains relatively stable with minor fluctuations and relatively stable elevations. The SMT migration amplitudes present a significantly positive linear correlation with SMT elevation changes, which are strongly controlled by the sediment supply. Following the SMT migration processes, both estuarine and bay beaches display clear erosion during the summer monsoon period that is mainly associated with strong river discharge and monsoon rainfall, respectively. In contrast, all beaches present distinct accretion and recovery during the winter monsoon period that are mainly due to the decreased monsoon rainfall and river discharge and/or the strengthened onshore sediment transport. Despite limitations, the results of this study have clarified the diverse SMT migrations of estuarine and bay beaches and aid in a better understanding of their possible morphodynamic responses. Unlabelled Image • SMT migration of estuarine beach has a significant difference from that of bay beach. • A landward SMT migration is linearly correlated with an increased SMT elevation. • Sediment supply drives the contrasting SMT migrations of estuarine and bay beaches. • All SMT-beaches present dominate erosion in summer and clear accretion in winter. [ABSTRACT FROM AUTHOR]

This study provides laboratory observations of the nearshore hydrodynamics and detailed topographic surveys associated with the morphological response of a beach-dune system to oblique waves during a storm event. A physical model consisting of a 20 m long sandy steep beach constructed in a wave basin, and a wave paddle oriented obliquely to the beach shoreline is used to generate irregular waves at the offshore boundary. In the experiment, the wave height is varied over time in five segments to simulate the change in the energy from a passing storm and is varied alongshore to initiate an alongshore gradient in waves and wave-driven currents. To simulate a constant storm surge, a high mean water level relative to the dune is used, enabling waves to reach the dune toe. The results indicate the formation and evolution of a beach scarp. The retreat of the scarp initially corresponds to time-varying wave energy, but becomes controlled by the changing nearshore beach slope over time. The alongshore current magnitude is governed by the spatio-temporal variability in wave energy and is the main driver of sediment flux. The sand that is eroded from the dune initially deposits at the dune toe when nearshore circulation is weak. However, as the wave energy increases sediment is transported along the beach and is deposited, increasing the surf zone width. The experimental results indicate that small waves at high water levels relative to the dune can drive the formation of a beach scarp. In addition, different levels of incident wave energy drive different rates of sediment transport and morphological evolution with high alongshore variability throughout a storm event. [ABSTRACT FROM AUTHOR]

Sandbanks can occur downdrift of headlands across embayments as a result of tidal currents and wave forcing, and may present a potentially valuable resource of marine aggregate material. Large sandbanks can alter tidal flows and, through refraction and dissipation, increase or decrease wave exposure along the shoreline of an embayment. The impact of offshore sandbanks on shoreline morphodynamics at varying timescales has not been rigorously investigated. Here we show, for the first time, that removing, lowering or raising a headland-associated sandbank can have a significant impact on longshore sediment transport. A Delft3D model of Start Bay and Skerries Bank, UK, validated on hydrodynamic observations, was used to conduct the numerical experiments. Results indicate that removing or lowering the bank generally reduces dissipation and increases wave heights, increasing longshore flux in the lee of the bank. Raising the bank generally has the opposite effect, increasing dissipation and lowering flux. Under oblique waves, the bank may protect the adjacent shoreline by refracting waves away from the coast. Shoreline flux for the region directly onshore of the bank acts as a continuous function of bank height (R 2 = 0.7), but correlations for distal regions are increasingly non-linear or absent. A raised bank would reduce transport rates to near-zero across extensive sections of the bay, reversing the long-term net flux at points distal (>5 km from bank) from the bank location, causing currently eroding regions to accrete. Our results demonstrate that moderate (≤5 m) elevation changes to a sandbank will likely produce significant variations in wave power at the shoreline, which will: (i) modify the shoreline response for individual storms; and (ii) substantially change shoreline morphology at decadal timescales. From a broader perspective, this study can be used to inform planning decisions involving naturally varying banks and/or potential mining of sandbanks, aiding shoreline protection strategies for locations with similar embayment-sandbank configurations. Unlabelled Image • First robust study of bank control on inshore waves and longshore transport • 20% change to bank elevation (≤5 m) significantly impacts longshore flux. • Directly in lee of bank, shoreline flux is a continuous function of bank depth. • Bank protects shore, dissipating direct waves and refracting away oblique waves. • Raised bank would substantially accrete distal (>5 km) currently eroding shoreline. [ABSTRACT FROM AUTHOR]